Anatomy & Physiology
Digestive System
The gut of nudibranchs consists of a mouth, buccal cavity, esophagus, stomach, intestine, rectum and anus (see Figure 1 below).
Figure 1. Internal morphology of the opisthobranch mollusc Austrodoris georgiensis adapted from Garcia et al., (1993).
Radula
A highly developed feeding apparatus (radula) is present in Platydoris scabra. The morphology of the radula is characteristic of the species. The radula of Platydoris scabra is relatively small (8 x 7 mm) however, contains a large number of teeth (Soliman 1978). The total number of teeth rows ranges from 40 – 75 (summarised by Soliman 1978). In the first row, the largest specimen examined by Soliman (1978), which was 10 cm long, had 34 teeth on each side of the naked rachis in the first row of teeth and 88 teeth in the fourth row. The number of teeth increased in each consecutive row. The teeth exhibit a variety of different shapes, and these are specially adapted for specific food types. In Platydoris scabra, the mid-lateral teeth have elongate yellowish hafts and terminal curved hooks (Figure 2, B). The outermost teeth are shorter and broader on both sides (Figure 2, A). Some teeth also have bifurcate hooks (Figure 2, C). Based on the relationship between the type of prey Platydoris species are associated with, and the morphology of the radula, it has been hypothesised that demosponges comprise majority of the diet of Platydoris species (Cattaneo-Vietti & Balduzzi 1991). For more information on the feeding behaviour of Platydoris scabra refer to the Life History and Behaviour link.
Figure 2. Morphology of the various teeth morphologies present in the radula of Platydoris scabra. Diagram adapted from Soliman (1978).
Reproductive System
To ensure the successful exchange of sperm between Platydoris scabra mating pairs, they have developed spines and hooks on their penises and vaginas to ensure that they remain securely together during copulation. This is especially useful in areas with high water flow. The penis has 11 horizontal rows (of 2-6 hooks which have a short hook-shaped cusp and a round base (Dorgan et al, 2002).
Figure 3. Morphology of the penal hooks in Platydoris scabra (scale bar = 150 μm). Photograph taken from Dorgan et al., (2002).
The reproductive organs of Platydoris scabra are illustrated in (Figure 4). During copulation, the sperm migrates to the receptaculum seminis then to the hermaphroditic duct (15 mm long) to be fertilized. The fertilised egg then travels along the hermaphroditic duct towards a convoluted ampulla (23 mm long) which is attached to the ventral side of the female glandular mass. The female gland mass functions much the same as the uterus in humans. Once inside the female glands, a nutritive layer is added to the fertilized egg, an outer capsule is added and once in the mucous gland, the eggs are bonded together in to a ribbon. For further information of the anatomy of Platydoris scabra’s reproductive organ refer to Soliman (1978).
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Figure 4. Morphologyof the reproductive system in Platydoris scabra including ampulla (a), prostate (pr), female glands (fg), bursa copulatrix (bc), receptaculum seminis (rs), deferent duct (dd), accessory gland (ag) and vagina (v). Scale bar = 1 mm. Diagram adapted from Dorgan et al.,(2002).
Coelomic systems
In molluscs, the coelom is reduced however the small coelom in molluscs functions in excretion and reproduction. In molluscs, the coelomic cavity is also known as the pericardial cavity (including the heart and the gonads).
Hemal System
Platydoris scabra has a hemal system consisting of a two chambered heart, a dorsal aorta, blood vessels and a hemocoel. The blood of molluscs contains amebocytes and respiratory pigement hemocyanin. The blood is transported to the gills for gas exchange. For further information on respiration in Platydoris scabra, refer to the Life History and Behaviour link.
Nervous system
As mentioned before in Physical description, Platydoris scabra has cephalic (head) tentacles and rhinophores to sense their environment (via touch, taste and smell). The two nerves in each oral tentacle pass directly to the brain (Murphy & Hadfield 1997). However, in the rhinophores, there are multiple nerves which converge on paired rhinophoral ganglia which connect to the cerebral ganglia (Murphy & Hadfield 1997). This enables communication environmental signals to the brain, which can then initiate a response. For example, when the two sensory organs detect chemical cues indicative of food, this message is passed to the brain, which then initiates a feeding response.
Excretory system
Wastes are excreted in all molluscs by the heart-kidney complex. The right kidney is lost in all opisthobranchs (Potts 1967). Therefore, excretion is accomplished by the left kidney only. The kidney is a large sac, which is surrounded by the blood of the hemocoel. The absorptive walls of the kidney allow it to exchange materials with the blood. Wastes such as ammonia and other nitrogenous compounds can then be incorporated into urine and removed from the body. For a further information regarding the excretory system in molluscs, refer to Potts (1967).
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